What is the point of a non-static data member const?

Question

For the following code:

class A
{
public:
    const int cx = 5;
};

Here, an instance of cx will be created for each object A. This seems unnecessary to me, since cx can never be modified. In fact, I see no reason why the compiler should not force a const element to be created. Can anyone explain this to me?

Answer 1

The const data element does not have to be the same for all instances. You can initialize it in the constructor.

class A
{
public:
    A(int n) :cx(n) {}

    const int cx;
};

int main()
{
    A a1(10);
    A a2(100);
}

Answer 2

Actually, I see no reason why the compiler should not have a static data element static.

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const , , , .

, :

struct Multiplier
{
    const int factor;

    Multiplier(int factor) : factor(factor) {}

    int operator()( int val ) const
    {
        return val * factor;
    }
};

std::vector<int> vec{1, 2, 3};
std::vector<int> vec2;

int i;
std::cin >> i;

std::transform( std::begin(vec), std::end(vec),
                std::back_inserter(vec2), Multiplier(i) );

// vec2 contains multiples of the values of vec

Answer 3

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class Matrix
{
    const int myRowCount;
    const int myColumnCount;
    std::vector<double> myData;
    //  ...
public:
    Matrix( int rows, int columns )
        : myRowCount( rows )
        , myColumnCount( columns )
        , myData( rows * columns )
    {
    }
    //  ...
};

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Answer 4

const.

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